This invention relates to a check-relief valve for a hydraulic circuit which is particularly but not exclusively designed for use in the header of a swather.
A swather header includes a table on which is mounted a first draper and a second draper. Each draper includes a draper canvas in the form of an endless loop and a plurality of support rollers for the canvas which form the loop into an upper run and a lower run with one or more of the rollers being driven to cause rotation of the roller about its axis and thus movement of the upper run of the draper canvas in a required direction longitudinal of the header.
Many such headers are arranged so that they can operate in different modes. In a first mode the first and second drapers are spread apart so that the material is fed on each draper toward a centre opening defined between the drapers through which the material falls to form the swath.
In an alternative mode, one of the drapers on a suitable support frame including the rollers and the canvas is moved longitudinally of the table so that the opening between the drapers is closed and a further opening is formed at one end of the header. In this mode, it is often desirable for the opening to be moved from one end of the header to the other end of the header and this is achieved by moving both drapers on suitable support frames longitudinally of the table.
As well as the lateral or linear movement of the draper assembly, it is of course also necessary when changing mode or changing the position of the opening to change the direction of rotation of the rollers driving the drapers. Both the actuator for moving the lateral movement of the draper support and the drive motor for rotating the rollers are generally powered hydraulically and it is most convenient to mount these devices in a single hydraulic circuit. Furthermore it is highly desirable that a single valve be provided which changes from one mode to another mode or changes the location of the draper and at the same time reverses the direction of the rotation of the draper. This single valve avoids any possible confusion or misfunction of the device and simplifies the operation for the driver.
The linear movement of the support for the draper can be actuated by a piston and cylinder arrangement or alternatively it can be carried out by a rotary motor together with a drive system which translates the rotary movement of the motor into linear movement of the support for example a lead screw arrangement.
In one arrangement which has previously been used, the actuator for moving the longitudinal movement and the drive motor for rotating the draper are simply mounted in parallel so that some of the fluid normally passing through the drive motor is bypassed into the actuator until the actuator moves the draper to an end stop at which time further fluid through the actuator is halted causing the whole of the fluid to return to its normal path through the drive motor. This device has been found to operate reasonably satisfactorily but in some cases additional friction in the support system carrying the draper can restrict the movement of the support so that the parallel fluid system does not provide enough pressure at the actuator to complete the movement quickly and positively. Free flow relief valves are available which can alleviate this problem. However these must be provided as a cooperating pair and in addition they are relatively expensive.